An induction heating (IH) type of image heating apparatus generates an eddy current through the action of a magnetic field generated by a magnetic field generation section upon a heat-producing medium. This image heating apparatus heat-fixes an unfixed image on recording paper such as transfer paper or an OHP sheet through Joule heating of the heat-producing medium by means of the eddy current.
This induction heating type of image heating apparatus can selectively heat only the heat-producing medium as compared with a heat roller type of image heating apparatus that uses a halogen lamp as a heat source, and therefore has the advantage of enabling heat production efficiency to be increased and the image heating apparatus startup time to be shortened.
It is desirable to use a thin heat-producing medium comprising a thin sleeve or endless belt as the heat-producing medium of this kind of image heating apparatus. That is to say, a thin heat-producing medium has low thermal capacity and can be made to produce heat in a short time. Therefore, an image heating apparatus that uses a thin heat-producing medium makes possible a marked improvement in startup responsiveness until heat production up to a predetermined heating temperature.
On the other hand, a heat-producing medium of low thermal capacity is prone to absorption of heat through the passage of recording paper and a drop in the temperature of the paper passage area.
Therefore, in an image forming apparatus using a thin heat-producing medium of this kind, the heat-producing medium is heated in a timely fashion to prevent the temperature of the heat-producing medium from falling below the predetermined heating temperature due to the passage of recording paper.
However, with an image heating apparatus that has this kind of configuration, if narrow recording paper is fed through continuously, the heat-producing medium that should suppress a drop in the temperature of the paper passage area is continually heated. Consequently, with this image heating apparatus, a paper non-passage area of the heat-producing medium may be subjected to an excessive rise in temperature.
The image heating apparatus disclosed in Unexamined Japanese Patent Publication No. HEI 10-74009 is known as an example of an image heating apparatus that eliminates this kind of rise in temperature of a paper non-passage area of the heat-producing medium.
FIG. 1 is an oblique drawing of an image heating apparatus disclosed in Unexamined Japanese Patent Publication No. HEI 10-74009.
As shown in FIG. 1, this image heating apparatus is equipped with a metal sleeve 1 as the above-described heat-producing medium that produces heat by means of induction heating, and a pressure roller 2 that exerts pressure on metal sleeve 1. Metal sleeve 1 is mounted on, and supported in a rotatable fashion by, the outer periphery of a cylindrical guide 7. A nip area (pressure area) through which recording paper 8 passes is formed between pressure roller 2 and metal sleeve 1 by the pressure of pressure roller 2 on metal sleeve 1.
This image heating apparatus is also equipped with an exciting coil 4 that generates a high-frequency magnetic field, and magnetic flux absorption members 6a and 6b that absorb magnetic flux. Exciting coil 4 is installed inside guide 7. Magnetic flux absorption members 6a and 6b are located on the outside of metal sleeve 1.
In FIG. 1, recording paper 8 bearing an unfixed toner image is transported in the direction indicated by the arrow S and fed into to the nip area. By this means, the unfixed toner image borne on recording paper 8 is heat-fixed onto recording paper 8 by the heat of metal sleeve 1 and the pressure between metal sleeve 1 and pressure roller 2.
In this image heating apparatus, recording paper 8 is basically transported on the right-hand side in FIG. 1, and if the width of recording paper 8 varies, the left-hand side in FIG. 1 is a paper non-passage area.
Magnetic flux absorption member 6b located on the left-hand side in FIG. 1 is configured so as to perform parallel movement in the axial direction along a level 5 through rotation of a motor 3.
When wide recording paper 8 is fed into the nip area, this magnetic flux absorption member 6b is moved to a position in which it is retracted from the paper passage area of this recording paper 8.
When narrow recording paper 8 is fed into the nip area, magnetic flux absorption member 6b is moved to the rear of magnetic flux absorption member 6a so as to be positioned in the paper non-passage area of this recording paper 8.
By this means, magnetic flux reaching the paper non-passage area of metal sleeve 1 from exciting coil 4 is absorbed by magnetic flux absorption member 6b, and is reduced.
Thus, in this image heating apparatus, by moving magnetic flux absorption member 6b according to the width of recording paper 8, magnetic flux arriving from exciting coil 4 is suppressed, and the rise in temperature in the paper non-passage area of metal sleeve 1 is reduced.
However, with this image heating apparatus, in order to perform parallel movement of magnetic flux absorption member 6b, the distance between movable magnetic flux absorption member 6b and metal sleeve 1 and the distance between magnetic flux absorption member 6a and metal sleeve 1 are different, as shown in FIG. 2.
Therefore, in this image heating apparatus, a difference tends to occur between the calorific value of the part of metal sleeve 1 opposite movable magnetic flux absorption member 6b and the calorific value of the part of metal sleeve 1 opposite magnetic flux absorption member 6a. 
Consequently, with this image heating apparatus it is difficult to heat the entire width of metal sleeve 1 uniformly.
FIG. 3 is an oblique drawing of another image heating apparatus disclosed in Unexamined Japanese Patent Publication No. HEI 10-74009. This image heating apparatus uses a magnetic flux masking shield as a means of reducing magnetic flux acting upon metal sleeve 1.
In FIG. 3, a magnetic flux masking shield 9 is positioned so as to be in line with the inner surface of a holder 10 between metal sleeve 1 and exciting coil 4.
When narrow recording paper 8 is passed through, magnetic flux masking shield 9 is moved to a position where it covers exciting coil 4 over an axial direction range equivalent to the paper non-passage area of metal sleeve 1.
On the other hand, when wide recording paper 8 is passed through, magnetic flux masking shield 9 is retracted to the outer edge of the paper passage width of metal sleeve 1.
Thus, in the image heating apparatus shown in FIG. 3, the entire width of metal sleeve 1 is heated uniformly when wide recording paper 8 is passed through.
In this image heating apparatus, since magnetic flux masking shield 9 is installed so as to be in line with the inner surface of holder 10 between metal sleeve 1 and exciting coil 4, magnetic flux masking shield 9 must be made thin.
However, when magnetic flux masking shield 9 is made thin, heat production due to induction heating increases. Moreover, holder 10 is generally made of a plastic material with low thermal conductivity.
Therefore, in the image heating apparatus shown in FIG. 3, there is little heat dissipation from magnetic flux masking shield 9 to holder 10, and there is a danger that magnetic flux masking shield 9 will continue to rise in temperature.
Furthermore, a problem with the image heating apparatuses shown in FIG. 1 and FIG. 3 is that a mechanism is necessary to perform parallel movement of magnetic flux absorption member 6b and magnetic flux masking shield 9, making the configuration of the overall apparatus complex and large.